Fornax_2019 initial spectra unit is different from other models

[Sheshuk]

Trying to make notebooks work in #402 I have the following problem.

In Fornax_2019.get_initial_spectra we have:

snewpy/python/snewpy/models/ccsn_loaders.py

Line 626 in 388312d

initialspectra[flavor] = _dLdE * self.fluxunit

where the flux_unit is defined in the c-tor:

snewpy/python/snewpy/models/ccsn_loaders.py

Line 300 in 388312d

self.fluxunit = 1e50 * u.erg/(u.s*u.MeV)

so the initialspectra entries are [1/s].

In all the other models we have initialspectra units ~ [1/(MeV s)].

As we're passing everithing to flux.Container, it checks the units compatibility and showed this issue.

Maybe it can be solved by a simple unit conversion? But I'm not very familiar with the internals of Fornax models, so I suggest to discuss this.